gotsmart--Funny you should ask that question.
Yesterday, I recalled that I had a product called PRI-G in my RV. It is recommend for gas tanks when the RV is in storage to keep the gas good. It also is recommended for boosting performance. On an impulse, I decided to add some to my gas tank. Almost immediately, I noticed more power. When I checked the mileage for that tank compared to mileage from the rest of the trip, it was 1 mpg better.
That was only 1 tank, so, I'll reserve final judgment until I use it over many tanks of gas. However, it is encouraging.